In general, an internal combustion engine has a fuel vapor processing apparatus for preventing fuel vapor generated in a fuel tank from being dissipated into the atmosphere. The fuel vapor processing apparatus guides the fuel vapor to a canister, where the fuel vapor is temporarily adsorbed, and causes an intake system of the internal combustion engine to take in the fuel vapor adsorbed in the canister via a purge valve together with fresh air introduced through a fresh air inlet.
In such a fuel vapor processing apparatus, if a crack is formed in a purge line that extends from the fuel tank to the purge valve through the canister, or if a seal failure occurs in the purge line, a leakage of the fuel vapor occurs. The expected dissipation prevention effect cannot be obtained.
There are known devices for processing fuel vapor. For example in Japanese Laid-Open Patent Application Publication No. (Tokkaihei) 6-173789, a leak check apparatus is used for checking whether or not the fuel vapor is leaking from the purge line based on a detected pressure change. This apparatus performs an operation called leak-down detection. In the leak-down detection, basically, the purge line is sealed after a predetermined negative pressure is applied thereto, and the presence or absence of a leakage is determined on the basis of a change in the inner pressure of the purge line. Since evaporation of fuel in the fuel tank continues after the purge line is sealed and the inner pressure changes accordingly, the leak detection accuracy is increased by correcting the pressure change during leak down on the basis of the generation speed of the fuel vapor, which is measured by performing an operation called vapor monitoring. In vapor monitoring, the inner pressure of the purge line is returned to atmospheric pressure after the measurement of change in the negative pressure during leak down, and then the purge line is sealed again to enable detection of an increase in the inner pressure from the atmospheric pressure.